The present invention relates generally to sensing signals using multiple pulses of electromagnetic radiation. In a specific embodiment, the invention relates more particularly to acoustic sensing using multiple optical pulses.
The telecommunications industry now uses optical fiber optic cables to form the vast majority of its network backbone. With advances in technology, a single cable bundle can carry many thousands or even millions of telephone conversations. With more recent demands for increased bandwidth for data and Internet traffic, the lack of redundancy within these networks has become a cause for concern. If a link within a network fails, there may be a significant cost to the network operator in both customer dissatisfaction and lost revenue. Such failures may occur, for example, when excavations associated with construction sever an optical fiber cable. Accordingly, the protection of these buried resources is a high priority for network operators. Practices have evolved to protect the fiber when work is scheduled in the vicinity of the fiber. However, unforeseen network failures still occur due to physical damage to the fiber plant.
A buried fiber alarm system that is able to detect and characterize acoustic signatures along the length of a fiber route would serve as a threat warning system for network operators. This would allow operators of the network to take action before critical failure, possibly avoiding damage to the cable entirely. It would also allow network traffic to be rerouted before service was lost. A sensor of this nature would also find application in many wide-ranging fields, such as structural monitoring and the protection of other vulnerable services such as oil and gas transmission pipelines. Furthermore, such a technology would be ideally suited to improvements in perimeter security. A security sensor of this nature would be unobtrusive and, if buried around the perimeter of a sensitive facility, would be virtually impossible to locate and disable. Certain optical sensor configurations would even remain operational if the fiber that is part of the sensor configuration were to be cut around the perimeter, allowing not only the detection of a cut, but also enabling determination of the location of the cut.
Much work has been done in the field of fiber optic based acoustic sensors. Perhaps the most sensitive techniques involve interferometric sensors. However, determining the location of the disturbance, and isolating a section of fiber from a persistent, non-threatening, disturbance, proves difficult due to the nature of these devices. Limited success has however been achieved using loop architectures, but due to the reciprocal nature of the loop configuration multiple disturbances of an unknown nature prove virtually impossible to separate and locate independently. This problem extends to any forward-propagating differential time-delay method.
Perhaps the most useful, truly-distributed, sensing technique employed in the field of fiber sensors is that of optical time-domain reflectometry (OTDR), a schematic representation of which is shown in FIG. 1. A number of varied methods and applications have been disclosed in the literature but the basic distributed scheme involves a short pulse of light, typically 10-1000 ns in duration, which is launched into a fiber, usually a single mode fiber. In FIG. 1, the pulse is launched from source 101, through isolator, 102, switch 103 and coupler 104. As the light propagates along the fiber under test (FUT), 108, a small fraction will be scattered by the tiny random fluctuations in the refractive index of the glass (scatter sites). Some of this scattered light is captured by the fiber and guided back toward the launch end of the fiber. This backscattered light, and its intensity as a function of time and hence distance along the fiber, can then be directly detected by, for example, a PIN diode detector, 105, through coupler 104. The backscattered signal may be recovered and displayed on oscilloscope 106. In the FIG. 1, 107 is a device or apparatus to prevent unwanted reflection from the unused port of the 3 dB coupler 104. FIG. 2 is a schematic representation of the backscattered signal showing a discontinuity, 201, associated with optical loss at a splice.
Other detection methods have been disclosed such as optical heterodyne, homodyne and optical amplification methods such as SOA (Solid-state optical amplification) and EDFA (Erbium doped fiber amplification). Unlike optical amplification techniques, optical heterodyne and homodyne methods require a coherent source and hence are termed coherent-OTDR or C-OTDR. Typically, the basic OTDR technique has found application in the measurement and characterization of waveguide features, such as the attenuation of the fiber, splice positions and loss, measurement of reflective markers and the certification of telecommunication installations. A representative C-ODTR technique is shown in FIG. 3. In this FIG, like items from FIG. 1 are labeled as in FIG. 1. The major difference in this technique is that a sample of the launch light and the backscattered light combine at the 3 dB coupler, 104, just before the detectors, 105, resulting in interference at the detectors, 105. A differential detector 109 may then be used to recover the backscatter intensity for exemplary analysis at oscilloscope 106.
In the mid 1980s it was noted that the use of a coherent excitation pulse (i.e. when the coherence length of the source is much greater than the pulse length Tc>L) in OTDR raised some interesting issues (Healey, P. “Fading in Heterodyne ODTR”, Electronic Letters, 20(1), pg 30, (1984)). It was noted that in the exemplary arrangement of FIG. 3 the backscattered trace is no longer a predictable, logarithmically falling signal, due to fiber loss, but that this predicted trace is modulated by a random variable. This random variable is due in part to the fact that the intensity of the light arriving on the detector at any specific time is the coherent addition of the light scattered from many discrete scatter sites. This “fading” mechanism is comparable to laser speckle, a random interference pattern caused by the interference of light scattering from different positions over the area of a spatially coherent beam. It was also noted that due to mechanical (such as vibration) and temperature changes, this random pattern is altered from pulse to pulse as the distribution of the scatter sites at a given location is also altered. This phenomenon was not exploited for sensing a change in the variables associated with the environment in which the fiber is located.
In U.S. Pat. No. 5,194,847 the same phenomenon is described, and is suggested for use in sensing of strain disturbances along the length of a standard single mode fiber, specifically for the detection of intrusion across a perimeter. In that patent there is described a system that generates a coherent pulse of light from a coherent source. This pulse is then directly launched along the fiber under test. The backscattered radiation is then detected by a square-law detection system, allowing the intensity of the backscattered signal to be observed. By detecting the change in this intensity for a given fiber section, information about the acoustic signal acting on the fiber can be recovered.
In U.S. Patent Publication 20050196174, “a method and apparatus is provided for obtaining status information from a given location along an optical transmission path. The method begins by generating a continuous wave (cw) probe signal having a prescribed frequency that is swept over a prescribed frequency range. The cw probe signal is transmitted over the optical path and a returned C-OTDR signal, in which status information concerning the optical path is embodied, is received over the optical path. A receiving frequency within the prescribed frequency range of the returned C-OTDR signal is detected to obtain the status information. The detecting step includes the step of sweeping the receiving frequency at a rate equal to that of the prescribed frequency. A period associated with the receiving frequency is temporally offset from a period associated with the prescribed frequency.” The fading problem discussed above remains in the disclosed method.
In published GB Patent Application 2222247A there is disclosed a “distributed fiber optic sensor system”. In the disclosed system, pulses of light which are shifted in relation to one another are transmitted along a fiber. A pulse of light having a first frequency is scattered or reflected from a first location along the optical fiber and combined, after guidance back to a detecting element, with light scattered by the second pulse from a second location along the optical fiber. In addition to the fact that this disclosure states that it involves the analysis of scattering from different sections of a fiber, the publication discloses only a single difference between the frequencies of the first and second pulses.
Although these techniques seem useful, there are several limitations in the disclosed systems. The most crucial limitation is reliability in detecting a threat, since missed detection would cause concern in many applications. The methods described above rely on a statistically random variable. Hence, at a given time, at any given position along the fiber under test, the signal recovered from the coherent addition of scattering from within the pulse has a finite probability of being close to zero. In a real world application this would leave this part of the fiber unprotected. Due to the natural slow drift of the environmental variables, this “faded” fiber section would eventually drift back to a situation where it would again return a signal; however the “black out period” is still a major concern. Another concern is that the signal returned from such a sensor is extremely non-linear and it may be difficult to identify an acoustic disturbance since the acoustic signature is distorted by the generation of harmonics associated with this non-linear response.
Accordingly, there is a need for an improved optical fiber, acoustic detection technique.